There are various systems which monitor natural and artificial environmental conditions for controlling them. For example, an air-conditioning system monitors the temperature and humidity in each room of building for controlling them at desired presettings. In such an air-conditioning system, the temperature and humidity of each room is monitored by sensors and the monitor results are transmitted to a central control unit at which a control signal for an air-conditioner in each room is generated for controlling the temperature and humidity in each room to desired values.
In this case, the method of transmitting the monitor result of the temperature and the humidity in each room to the central control unit includes a technique using an existing private branch exchange (PBX) (refer to Patent document 1). Specifically, sensors are mounted on a telephone set in each room, so that monitor results of the sensors are transmitted to a computer in the central control unit via the telephone set, PBX and a modem.
There is also a system which detects the damages of a protective net for preventing rocks from falling which is provided on a slope along a road, so that they are alarmed to remote a control room (refer to Patent document 2). In this system, IC tags having displacement sensors incorporated therein are mounted on the falling rock preventive net or rocks, so that displacement data are wirelessly transmitted to the control room. An example in which a cellular phone is used as a wireless transmitter is disclosed.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-A-7-158940, pages 3 and 4, FIG. 1
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-P2002-228497A, pages 2 and 3, FIG. 1